1. Field of the Invention
The present invention is directed to a safety coupling device between a drive shaft and an axial aligned driven shaft of a press which device will automatically disengage in response to an overloading torque being applied thereto.
2. Prior Art
In presses which operate on a workpiece such as a platen press which die cuts a sheet of paste board, the sheet of material is transferred from one station to another by a conveyor which utilizes a pair of endless chains carrying spaced gripper bars which chains are driven by a pair of sprocket wheels mounted on a shaft. To prevent damage to the conveyor due to jamming of the conveyor or portions of the press, an overload release coupling or clutch is provided between the drive shaft and the driven shaft of the conveyor which coupling will become disengaged at a predetermined amount of an overload torque to allow slippage to occur between the drive shaft and the driven shaft. An example of such an overload release coupling is disclosed in U.S. Pat. No. 3,063,263 which issued to Bobst et al on Nov. 13, 1962.
In the coupling disclosed in the Bobst et al patent, a disk is coupled to the drive shaft for rotation therewith and has a plurality of protrusions extending from both sides with the protrusions being on the circle which is concentric with the shaft. The housing connected to the driven shaft for rotation therewith encloses the disk and protrusions and has a plurality of cylinders receiving spring biased pistons or yieldable members which have surfaces for engaging each of the protrusions of the disk. When an overload occurs, the springs allow the pistons to move away from engagement with the protrusions, which are preferably balls carried by the disk, to allow slippage between the disk and the housing.
One of the shortcomings of the coupling disclosed in the Bobst et al patent is that contact errosion or wear occurs at the surfaces of the ball and the engaging surfaces of the pistons. Due to the pressure and friction acting on the surfaces of the pistons and the balls during transmission of torque between the disk and housing, contact wear cannot be avoided. A second problem or disadvantage with the clutch or coupling of the above mentioned Bobst patent is the provision of a plurality of cylinders with pistons engaging opposite sides of the protrusions on the disk involves accurate machining operations which increase the manufacturing costs for providing the clutch or coupling cylinders, pistons and individual springs.